P-N semiconductors

Intrinsic semiconductors

Si and Ge (fixed band gap or pure element). They are excited by heat or light, no doping

Doping

Pure (intrinsic) semiconductors conduct poorly.
To improve conductivity, we dope them with tiny amounts of impurities.

n-type

• Dope silicon with a group 15 element (P, As, Sb)

• These atoms have 5 valence electrons

• One extra electron is left over

• That electron is easily excited into the conduction band

p-type

• Dope silicon with group 15 element

• 3 bonds form, one bond is missing an electron

• This creates a hole

• Gives space for an electron to jump to the valence band

P-N junction

When n-type and p-type materials touch without voltage, holes go to the n side and electrons go to the p side, creating a depletion region. This is used for diodes, transistors, solar cells

Temperature in semiconductors

Increasing temperature increases conductivity in semiconductors because it gives energy to the electrons to jump to the conductivity band.

Temperature in conductors

High temperature lowers conductivity in conductors because it creates a disturbance in the flow of free electrons within the lattice.

Why do metals show conductivity

Their valence and conduction band overlap

Why do insulators not show conductivity

The band gap is too large

Band gap

The HOMO–LUMO gap is the energy needed to excite an electron

Extrinsic semiconductors

Doped semiconductor Si:P (phosphorus-doped silicon)